1. Field of the Invention
The present invention relates to a charge pump MOS voltage booster and to two applications where said type of booster can find advantageous use.
2. Discussion of the Related Art
The increasing need for devices which operate in equipment having one continuous very low power voltage (down to 1V), such as telecommunications line equipment, portable units, etc., requires efficient and simple continuous voltage boosters.
One well known structure of a charge pump doubler is illustrated in FIG. 1. It includes an oscillator OSC typically with square wave and powered with a continuous power voltage VS and connected to ground GND and having an output O. The output O is connected to the first terminal of a first charge transfer capacitor TC1. The second terminal of the charge transfer capacitor TC1 is connected to the cathode of a diode D2. The anode of the diode D2 is connected to the power voltage VS. The cathode of the diode D2 is also connected to the anode of another diode D1. The cathode of the diode D1 is connected to the output OUT of the doubler and to the second terminal of a charge accumulation capacitor SC whose first terminal is connected to ground GND.
In this circuit the output voltage (without load) is equal to double the power voltage decreased by double the starting voltage (approximately 0.7V) of the diodes D1 and D2. When the power voltage is very low, e.g. between 1.2V and 3.5V, this reduction becomes significant and unacceptable.
To solve this problem it has bee proposed, e.g. in French patent application FR-A-2 321 144, to replace the diodes D1 and D2 with two MOS transistors M1 and M2 as shown in FIG. 2. Naturally threshold transistors M1 and M2 need to be appropriately piloted. This was achieved, as shown in said document, by means of two other MOS transistors M3 and M4.
The circuit of FIG. 2 solves the problem of voltage drop on the diodes since the voltage drop on the channel of the MOS transistors is extremely small but exhibits, as also the circuit of FIG. 1, a certain ripple at the output OUT.